legacy: build options refinements

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Slight simplification of the builder's BuildOptions struct.

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (c004b42) 63.10% compared to head (29bf461) 63.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2343      +/-   ##
==========================================
- Coverage   63.10%   63.02%   -0.09%     
==========================================
  Files         201      201              
  Lines       19262    19260       -2     
==========================================
- Hits        12155    12138      -17     
- Misses       6056     6067      +11     
- Partials     1051     1055       +4     

Flag	Coverage Δ
unit	63.02% <50.00%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
arduino/builder/builder.go	71.42% <100.00%> (ø)
arduino/builder/build_options_manager.go	81.72% <46.15%> (-0.39%)	⬇️

... and 2 files with indirect coverage changes

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[cmaglie]

After discussing this PR with my colleagues, we decided to not merge it and use a different approach.